The present invention relates to a method for forming a latent image on an image carrier in an image forming apparatus, such as an electronic copying machine or printer.
In copying a document or original on an electronic copying machine, for example, a latent image of the original is formed on the surface of a photosensitive member in the copying machine. Conventional methods of latent image formation include the following system. First, as shown in FIG. 1, the surface of a photosensitive member 2 is substantially and uniformly charged by a charging unit 4. Then, a light beam 6 reflected by an original is projected on the charged surface of the photosensitive member 2 to form an electrostatic latent image thereon. Thereafter, toner is electrostatically attached to the electrostatic latent image by a developing unit 8 to develop the image. Namely, the conventional image forming method comprises a charging process for charging the photosensitive member 2, an exposure process for applying a light beam to the charged portion of the photosensitive member 2 to form an electrostatic latent image thereon, and a developing process for developing the electrostatic latent image, these processes being executed in the order named. This prior art method is known as an image forming process of the Carlson electrophotographic method.
According to the prior art method, however, the electric charges applied to the photosensitive member 2 in the charging process need to be held on the photosensitive member's surface until the developing process following the exposure process is reached. Therefore, the electric charges on the photosensitive member 2 must be maintained for a relatively long time. Thus, requiring a long interval between the charging and developing processes, the conventional method is subject to the following drawbacks:
1. If the electric charge holding capability of the photosensitive member is low, then it is necessary to previously supply a high voltage to the photosensitive member is consideration of a reduction of potential. Thus, the charging unit requires a large capacity.
2. If too high a voltage is previously supplied to the photosensitive member in consideration of the reduction of potential, the photosensitive layer will possibly be electrically broken.
3. Since the material of the photosensitive member needs to be high in dark resistance (approx. 10.sup.13 to 10.sup.14 .OMEGA..multidot.cm) for a prolonged hold of electric charges, it must be selected from limited kinds of materials.
Some photosensitive materials, not so high in resistance, cannot be used for this purpose, although they are excellent in sensitivity or in mechanical hardness. For example, some amorphous silicon materials are too low in dark resistance and, hence, in electric charge holding capability to be conventionally used as effective photosensitive materials for the purpose concerned.